It is well known that α-Cr2O3 and α-Fe2O3 have in common the structure of α-Al2O3 (corundum) with the M+3 ions occupying octahedral sites in the hexagonally close-packed oxide lattice. In contrast, CoO has a lattice similar to NaCl while CO3O4 has a normal spinel structure. These basic structures are described in standard treatises; see, for example, pages 538, 543–545, and 550 of Structural Inorganic Chemistry by A. F. Wells, 5th ed. Clarendon Press, Oxford, UK (1986). γ-Chromium oxide (CrO2.44) is described in Wilhelmi, Acta Chemica Scandinavica, Vol. 22, pages 2565–2573 (1968).
Numerous mixed metal oxides have been prepared in which the cation sites of the lattice are occupied by different metal ions. For example, solid solutions of the type (CrxFe1-x)2O3 are known where 0<x<1. These materials have been prepared by standard ceramic or sol-gel techniques as described by Music, et al. in J. Materials Science, Vol. 31, pages 4067–4076 (1996) and by Bhattacharya, et al. in J. Materials Science, Vol. 32, pages 577–560 (1997).
Mixed Cr—Co oxides having a spinel structure are known (see e.g., Bracconi et al. in Ann. Chim. Fr., Vol. 4, pages 331–338 (1979) and Hanck and Laitinen in J. Inorg. Nucl. Chem., Volume 33, pages 63–73 (1971)).
CrCoO3 is referenced as an interconnector material in a fuel cell assembly (see Chem. Abs. 118:9397). Various mixed metal oxides containing cobalt and chromium are also disclosed in Castiglioni, et al., J. Solid State Chemistry, Vol. 152, 526–532 (2000); Nowotny et al., J. Am. Ceram. Soc., Vol. 65, pages 192–196 (1982); and Zhang et al., Journal of Power Sources, Vol. 83, pages 121–127 (1999).
Certain metal oxides are used as catalysts and/or catalyst precursors in the manufacture of fluorinated hydrocarbons. Chromium(III) oxide in particular is useful as it has been found that it may be fluorinated by HF at elevated temperature to a give mixture of chromium fluoride and chromium oxyfluoride species which are active catalysts for conversion of C—Cl bonds to C—F bonds in the presence of HF. This conversion of C—Cl bonds to C—F bonds by the action of HF, known generally as halogen exchange, is a key step in many fluorocarbon manufacturing processes.
Chromium oxide compositions useful as catalyst precursors may be prepared in various ways or may take various forms. Chromium oxide suitable for vapor phase fluorination reactions may be prepared by reduction of Cr(VI) trioxide, by dehydration of Guignet's green, or by precipitation of Cr(III) salts with bases (see U.S. Pat. No. 3,258,500). Another useful form of chromium oxide is hexagonal chromium oxide hydroxide with low alkali metal ion content as disclosed in U.S. Pat. No. 3,978,145. Compounds such as MF4 (M=Ti, Th, Ce), MF3 (M=Al, Fe, Y), and MF2 (M=Ca, Mg, Sr, Ba, Zn) have been added to hexagonal chromium oxide hydroxide to increase catalyst life as disclosed in U.S. Pat. No. 3,992,325. A form of chromium oxide that is a precursor to a particularly active fluorination catalyst is that prepared by pyrolysis of ammonium dichromate as disclosed in U.S. Pat. No. 5,036,036.
The addition of other compounds (e.g., other metal salts) to supported and/or unsupported chromium-based fluorination catalysts has been disclosed. Australian Patent Document No. AU-A-80340/94 discloses bulk or supported catalysts based on chromium oxide (or oxides of chromium) and at least one other catalytically active metal (e.g., Mg, V, Mn, Fe, Co, Ni, or Zn), in which the major part of the oxide(s) is in the crystalline state (and when the catalyst is a bulk catalyst, its specific surface, after activation with HF, is at least 8 m2/g). The crystalline phases disclosed include Cr2O3, CrO2, NiCrO3, NiCrO4, NiCr2O4, MgCrO4, ZnCr2O4 and mixtures of these oxides. Australian Patent Document AU-A-29972/92 discloses a mass catalyst based on chromium and nickel oxides in which the Ni/Cr atomic ratio is between 0.05 and 5. U.S. Patent Application Publication No. 2001/0011061 A1 discloses chromia-based fluorination catalysts (optionally containing Mg, Zn, Co, and Ni) in which the chromia is at least partially crystalline. Fluorinated catalysts containing cobalt and chromium in combination (e.g. impregnated on a support) are among those disclosed in U.S. Pat. No. 5,185,482. U.S. Pat. No. 5,559,069 discloses homogeneously dispersed multiphase catalyst compositions characterized by dispersed phases of certain divalent metal fluorides (certain fluorides of Mn, Co, Zn, Mg, and/or Cd) and certain trivalent metal fluorides (fluorides of Al, Ga, V, and/or Cr).
There remains a need for halogen exchange catalysts that can be used for processes such as the selective fluorination and chlorofluorination of saturated and unsaturated hydrocarbons, hydrochlorocarbons, hydrochlorofluorocarbons, and chlorofluorocarbons, the fluorination of unsaturated fluorocarbons, the isomerization and disproportionation of fluorinated organic compounds, the dehydrofluorination of hydrofluorocarbons, and the chlorodefluorination of fluorocarbons.